Pollution control devices are employed on motor vehicles to control atmospheric pollution. Two types of such devices, catalytic converters and diesel particulate filters or traps, are currently in widespread use. Catalytic converters contain a catalyst, which is typically coated onto a monolithic structure mounted in the converter. The monolithic structures (termed “monoliths”) are typically ceramic, although metal monoliths have been used. The catalyst oxidizes carbon monoxide and hydrocarbons, and reduces the oxides of nitrogen in automobile exhaust gases to control atmospheric pollution. Diesel particulate filters or traps are generally wall flow filters which have honeycombed monolithic structures (also termed “monoliths”), typically made from porous crystalline ceramic material.
Typically, as constructed, each type of these devices has a metal housing which holds within it a monolithic structure or element that can be metal or ceramic, and is most commonly ceramic. The ceramic monolith generally has very thin walls to provide a large amount of surface area and is fragile and susceptible to breakage. Moreover, there is a current industry trend regarding ceramic monoliths used in automotive catalytic converters toward monoliths having smoother outer surfaces that makes it more difficult to maintain adhesion of the mounting material to the ceramic monolith during canning. Ceramic monoliths also typically have coefficients of thermal expansion that are an order of magnitude less than typical metal housings (for example, stainless steel housings) in which they would be contained in use. To avoid damage to the ceramic monolith from road shock and vibration, to compensate for the thermal expansion difference, and to prevent exhaust gases from passing between the monolith and the metal housing, ceramic mat or intumescent sheet materials are often disposed between the ceramic monolith and the metal housing.